This invention relates to exhaust timing control for an internal combustion engine, and more particularly, to exhaust timing control for a two-cycle engine of a watercraft for producing optimal watercraft acceleration at all engine speeds and watercraft velocities.
The xe2x80x9cpersonal watercraftxe2x80x9d has become a very popular and growing segment of the watercraft market. This type of watercraft is comprised of a relatively small hull that defines a rider""s area which is designed primarily to accommodate a rider and possibly one or two additional riders. Due to the space limitations and power demands of these watercraft, a two-cycle engine is typically provided to drive these watercraft.
As illustrated by curves A and B of FIG. 1, it has been recognized in the past that the maximum output of an engine can be increased by advancing the closing of the exhaust port. The cost of increasing the output by advancing the closing of the exhaust port is that an engine output trough occurs in the middle engine speed range, as illustrated in curve B. This output trough presents a problem because it tends to occur prior to the planing of the watercraft and at the same time the watercraft exhibits a crest in hull resistance. As a result of this overlapping output trough and hull resistance crest, a difficult-to-accelerate area occurs prior to planing.
Engine output troughs at low and medium engine speed ranges have been eliminated in the past by varying exhaust port timing through a variable exhaust control valve. At low and medium engine speed ranges the valve is moved to a position so that the closing of the exhaust port is advanced. By advancing the closing of the exhaust port in the low and medium engine speed ranges, the compression ratio in the engine is increased where power is needed the most. At high engine speeds, the valve is moved to a position so that the closing of the exhaust port is relatively delayed. By delaying the closing of the exhaust port in the high speed range, the compression ratio in the engine is decreased. A decreased compression ratio in the engine at high engine speeds is important for maintaining appropriate temperature control, avoiding excess pressures in the combustion chamber and avoiding pre-ignition and knocking in the engine. Variable exhaust control valve arrangements of the past would vary exhaust port timing by advancing or delaying the closing of the exhaust port based on pre-determined engine conditions, such as engine speed or exhaust pressure.
Although a variable exhaust control valve arrangement that varies exhaust port opening based solely on pre-determined engine conditions may be suitable for some vehicle uses, it is not ideal for a watercraft. A watercraft is unique in that the engine that propels the watercraft must overcome a large amount of hull resistance in the low and medium watercraft velocities before the watercraft reaches a planing velocity. By varying the exhaust port opening based on watercraft traveling conditions and pre-determined engine conditions, the engine is more responsive to the performance needs of the watercraft so that watercraft exhibits smoother acceleration as engine speed increases, especially in the low and medium watercraft velocity and engine speed ranges prior to planing.
It is therefore a primary object of the present invention to provide a personal watercraft with an engine that includes an exhaust control valve arrangement that varies exhaust port timing based on engine and watercraft traveling conditions.
Although these exhaust control valves are extremely effective in improving engine performance in a watercraft, they are susceptible to salt corrosion and sticking when the watercraft is used in a sea water environment. As is well known with many types of watercraft, exhaust gases from the watercraft engine are discharged through an exhaust system to the atmosphere either through the water, or near the water, in which the watercraft is operating. Salt may accumulate on the exhaust control valve caused by sea water entering the exhaust control system when the watercraft is capsized and righted or through marine air entering the induction system. Corrosion and sticking may occur between the valve and a respective sliding surface. This sticking prevents the exhaust control valve from operating in the manner intended.
It is therefore a further object of this invention to provide an improved exhaust system for a personal watercraft that prevents sticking and corrosion of the exhaust control valve caused by salt from sea water and air.
A two-cycle internal combustion engine for a watercraft that includes a cylinder block having a cylinder bore, a piston reciprocating in the cylinder bore and an exhaust port formed in the cylinder block. An exhaust path extends from the cylinder bore for exhausting combustion products from the cylinder bore. An exhaust control valve cooperates with the exhaust port and is movable between at least a first position for advancing the closing of the exhaust port so as to increase the compression ratio in the engine and a second position for delaying the closing of the exhaust port so as to decrease the compression ratio in the engine. Means for controlling the exhaust control valve between the first and second position are provided and are dependent upon a watercraft condition.